Granulite Facies Metamorphism Research Articles

Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway)

Graphite formation in the deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten–Vesterålen Complex, northern Norway. Graphite schist is hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite (<modality 39%), quartz, plagioclase, pyroxenes, biotite (Mg# = 0.67–0.91; Ti < 0.66 a.p.f.u.) and K-feldspar/perthite. Pyroxene is orthopyroxene (En69–74) and/or clinopyroxene (En33–53Fs1–14Wo44–53); graphite occurs in assemblage with metamorphic orthopyroxene. Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810–835 °C and 0.73–0.77 GPa; Zr-in-rutile thermometry 726–854 °C. COH fluids stabilise graphite and orthopyroxene; the high Mg#-ratio of biotite and pyroxenes, and apatite Cl < 2 a.p.f.u., indicate the importance of fluids during metamorphism. Stable isotopic δ13Cgraphite in the graphite schist is −38 to −17‰; δ13Ccalcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ13Ccalcite = −8.7‰ to −9.5‰ and heavier values for δ13Cgraphite = −11.5‰ to −8.9‰. δ18Ocalcite for marble shows lighter values, ranging from −15.4‰ to −7.5‰. We interpret the graphite origin as organic carbon accumulated in sediments, while isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration.

Protracted localization of metamorphism and deformation in a heterogeneous lower-crustal shear zone

Deformation of the continental crust is influenced by the pre-existing structural framework, fluid availability, strain rate, and pressure-temperature conditions. We investigate the evolution of a large (hundreds-of-meters wide) heterogeneous shear zone and associated brittle and ductile deformation structures (Lofoten, Norway) using structural analysis, mineral-chemical and microstructural observations, and U-Pb dating. The shear zone developed through a sequence of metamorphic stages: (1) migmatization and granulite-facies metamorphism, (2) eclogite-facies metamorphism, and (3) amphibolite-facies metamorphism. Stage (1) was related to magmatic activity at lower-crustal conditions prior to 1.7 Ga. Stage (2) likely occurred during the early phases of the collision between Baltica and Laurentia (Caledonian collision). Stage (3) was related to shortening during the main phase of the Caledonian collision (∼430–400 Ma) and was accompanied by hydration of the shear zone. We demonstrate how mechanical heterogeneities influence the deformation style from the centimeter to meter scale. Zones with a pre-existing fabric deformed by ductile shearing and folding, whereas homogeneous, dry rocks fractured. Fractures provided precursors for small-scale shear zones. These contrasting deformation styles occurred repeatedly. Consequently, pre-existing structures define the deformation style and serve as conduits for the channelization of fluids over extended periods of time.

Exploring paleoproterozoic metamorphism in the western Río de la Plata craton from drill-core evidence

Rhyacian basement is not exposed in the western part of the Río de la Plata craton, but deep borehole samples (depth of 1046–3340 m) provide information on its composition and metamorphic evolution. An amphibole schist sample from the Camilo Aldao borehole underwent two metamorphic stages: M1 and M2. The P-T conditions of M1 are not constrained whereas the M2 main metamorphic stage is characterized by conditions of 6.1–7.4 kbar and 575–605 °C. In contrast, an olivine gabbronorite sample from the Santiago Temple borehole records crystallization conditions of 1.7 ± 0.6 kbar and 1180 ± 10 °C based on clinopyroxene composition, and late magmatic to subsolidus low-pressure granulite facies metamorphism at 2.3 ± 1.2 kbar and 749 ± 85 °C based on multi-equilibrium thermobarometry. The Rhyacian evolution of the Río de la Plata craton is characterized by at least three thermo-tectonic orogenic stages: T1 (ca. 2200–2100 Ma), T2 (ca. 2100–2040 Ma), and T3 (ca. 2040–2010 Ma) that correspond respectively to: 1) early oceanic subduction and juvenile continental arc magmatism; 2) subsequent continental collision (clockwise intermediate P/T paths of metamorphism), and 3) final low-pressure metamorphism coeval with post-orogenic magmatism. The Camilo Aldao amphibole schist is representative of the T2 orogenic stage while the Santiago Temple gabbronorite is representative of T3 magmatism and metamorphism.

Zircon U–Pb geochronology, Nd isotopes and geochemistry of mafic granulites from the Central Indian Tectonic Zone: isotopic constraints on Proterozoic crustal evolution

Abstract The Central Indian Tectonic Zone (CITZ) comprises northern and southern Indian cratonic blocks and is a tectonic window that is suitable for investigating the Proterozoic crustal evolution because of the presence of a wide variety of lithologies. Geochemical and geochronological data on mafic granulites by previous workers do not ascertain the possibility of mafic protoliths and their coeval link to other CITZ units. Thus, determining the precise timing of the formation of mafic granulites may indicate a connection between metamorphism and fragmentation of the Columbian supercontinent. This study presents zircon U–Pb ages, Nd isotopes and the geochemistry of mafic granulites to evaluate their genesis and timing of metamorphism. The results show the tholeiitic affinity and primary magmatic differentiation of the parental melt. Depletion of Nb, P, Zr and Ti and positive enrichment of Ba, U and Pb indicate the derivation of mafic granulites from a variably enriched subcontinental lithospheric mantle (SCLM) source. The zircon U–Pb ages (1564 ± 8 to 1598 ± 9 Ma) are interpreted as a period of granulite-facies metamorphism. The T DM (depleted-mantle) model ages (2.9–3.4 Ga) of mafic granulites indicate the timing of mafic protolith extraction. The mineral isochron age c. 1.0 Ga indicates that these rocks underwent some events during an early Neoproterozoic period. Protolith of mafic granulites could be related to the evolution of melts derived from metasomatized SCLM through fractional crystallization processes.

Late Paleoproterozoic ultrahigh-temperature mafic granulite from the eastern Alxa Block, North China Craton

The timing and mechanism of ultrahigh-temperature (UHT) granulite facies metamorphism is critical to understanding the tectono-thermal evolution of granulite terranes and their relationships to supercontinent assembly. Here we document late Paleoproterozoic UHT mafic granulites from the Diebusige Complex in the eastern Alxa Block, North China Craton. These rocks occur as lens within felsic gneiss, marble, and garnet-bearing pyroxenite and contain representative granulite-facies minerals such as garnet, clinopyroxene, plagioclase, and Ti-rich amphibole. Based on phase equilibria modelling, Ti-in-amphibole (Amp) thermometer, and rare earth element (REE)-based thermometers, UHT metamorphic condition of 948 – 1048 ℃/11 – 14 kbar was obtained for the peak metamorphic stage, which was followed by retrograde metamorphism under lower amphibolite – greenschist facies (401 – 565 ℃/5.2 – 7.1 kbar). Zircon U-Pb dating and trace element systematics suggest that the protolith of the studied UHT mafic granulites emplaced at ∼ 1.97 Ga. This is followed by rapid metamorphism to UHT conditions and slow cooling in the lower crust during 1.96 – 1.86 Ga, suggesting a long-lived hot orogen in the eastern Alxa Block, possibly resulting from Andean-type subduction – accretion in the eastern Alxa Block. Combining chronological and metamorphic P-T data on UHT metamorphism from adjacent areas, our results support a long-lived late Paleoproterozoic (ca. 2.0 – 1.8 Ga) hot orogen that extends at least from the Khondalite Belt to the eastern Alxa Block.

Paleozoic Tectonothermal Evolution in the West Qinling Orogen, Central China: Petrological and Chronological Evidence from Garnet Amphibolites

The Qinling Complex is located in the core of the northern Qinling Orogen and plays a key role in understanding the tectonic evolution of the Qinling Orogen, but its metamorphic evolution remains controversial. The combined investigation of petrographic observation, zircon U-Pb dating, and phase equilibria modeling for garnet amphibolites from the Tianshui area in the West Qinling Orogen is reported in this study. The results show that the garnet amphibolites record a clockwise P-T path characterized by a pre-TMax decompression heating stage, a temperature peak at P-T conditions of 0.84–0.99 GPa and 869–886 °C, followed by a decompression cooling stage. Zircon U-Pb dating yields four age populations of ~479 ± 4 Ma, ~451 ± 8 Ma, ~411 ± 4 Ma, and ~377 ± 6 Ma. The 479–450 Ma reflects the timing of the pre-TMax high–medium pressure upper amphibolite-facies metamorphism. The metamorphism at peak temperature condition occurred at c.411 Ma and was followed by decompression cooling to c.377 Ma. The Ordovician high–medium pressure metamorphism is related to the continental collision, which is slightly later than the HP–UHP eclogite-facies metamorphism in the East Qinling Orogen. The HT granulite-facies metamorphism at peak temperature condition took place at reduced pressures, suggesting thinning of the collision-thickened orogenic crust. Therefore, the northern West Qinling Orogen experienced a tectonothermal evolution from initial crust thickening to thinning during the Paleozoic collisional orogeny.

Late Mesoproterozoic to early Neoproterozoic metamorphic history of the North Wulan terrane, NW China: New constraints on the evolution of Rodinia

Late Mesoproterozoic to early Neoproterozoic metamorphic and magmatic events related to the assembly of Rodinia are identified in microcontinents (such as North Wulan terrane, Qaidam block, Central Qilian terrane, Quanji Massif, and Hualong terrane) from the northeastern Tibet Plateau. However, the peak P–T conditions, P–T paths, and timing of the metamorphic events are still controversial. In this contribution, we integrate petrography, phase equilibrium modelling, mineral chemistry and zircon geochronology to develop a detailed metamorphic history of felsic paragneiss and garnet amphibolite from the North Wulan terrane. Zircon U–Pb geochronology reveals two stages of metamorphism at 1.2–1.0 Ga and ∼0.9 Ga. The earlier metamorphic event caused granulite-facies metamorphism with peak P–T conditions of 800–850 °C at 8–10 kbar. The second metamorphic event (∼0.9 Ga) reached amphibolite-facies conditions at 700–750 °C and 6–7 kbar. Granulite-facies metamorphism probably took place in an arc-related setting, whereas amphibolite-facies metamorphism was likely associated with tectonic extrusion during the collision between the North Wulan terrane and unknown continental fragments. The North Wulan terrane and surrounding continental blocks/fragments in the northeastern Tibet Plateau share a similar evolution that includes long-lived oceanic subduction from 1.2 to 1.0 Ga followed by continental collision at ∼0.9 Ga at the periphery of Rodinia. Our results improve the understanding of the tectonic processes responsible for microcontinent evolution in the northeastern Tibet Plateau during the assembly of Rodina, and provide important constraints on the reconstruction of Rodinia.

Petrochemistry of Charnockitic Rocks of Awo-Osuntedo Area, southwestern Nigeria

The petrochemistry of charnockitic rocks from the Awo-Osuntedo area of southwestern Nigeria were investigated with the objectives of presenting their petrogenesis and geotectonic setting at the time of their emplacement. The rocks were mainly found within the migmatitic gneiss country rock and associated pegmatites. They occurred as small intrusive bodies and are composed mainly of orthopyroxene, clinopyroxene, quartz, biotite, secondary amphibole, plagioclase and alkaline feldspars with a small compositional range between basic and intermediate respectively for Awo and Osuntedo rocks. The small Eu* anomalies (0.9-1.47) with an average of 1.2 combined with elemental signatures indicate that the rocks were derived from an enriched mantle with some crustal contamination and emplaced in an environment similar to the arc region of tectonic setting. Though, the rocks have dry mineral assemblages, there is no geochemical evidence to suggest they were derived from melting that was associated with granulite facies metamorphism rather the small sizes of the plutons and the geochemical signatures particularly high magnesian and calc-alkaline nature are typical of the Caledonian type charnockite formed during delamination of thickened continental crust after collisional orogeny.

Petrochronologic constraints on inverted metamorphism, terrane accretion, thrust stacking, and ductile flow in the Gneiss Dome belt, northern Appalachian orogen

AbstractGneiss domes are an integral element of many orogenic belts and commonly provide tectonic windows into deep crustal levels. Gneiss domes in the New England segment of the Appalachian orogen have been classically associated with diapirism and fold interference, but alternative models involving ductile flow have been proposed. We evaluate these models in the Gneiss Dome belt of western New England with U‐Th‐Pb monazite, xenotime, zircon, and titanite petrochronology and major and trace element thermobarometry. These data constrain distinct pressure–temperature–time (P‐T‐t) paths for each unit in the gneiss dome belt tectono‐stratigraphy. The structurally lowest units, Laurentia‐derived migmatitic gneisses of the Waterbury dome, document two stages of metamorphism (455–435 and 400–370 Ma) with peak Acadian metamorphic conditions of ~1.0–1.2 GPa at 750–780°C at 391 ± 7 to 386 ± 4 Ma. The next structurally higher unit, the Gondwana‐derived Taine Mountain Formation, records Taconic (peak conditions: 0.6 GPa, 600°C at 441 ± 4 Ma) and Acadian (peak: 0.8–1.0 GPa, 650°C at 377 ± 4 Ma) metamorphism. The overlying Collinsville Formation yielded a 473 ± 5 Ma crystallization age and evidence for metamorphic conditions of 650°C at 436 ± 4 Ma and 1.2–1.0 GPa, 750–775°C at 397 ± 4 to 385 ± 6 Ma. The structurally higher Sweetheart Mountain Member of the Collinsville Formation yielded only Acadian zircon, monazite, and xenotime dates and evidence for high‐pressure granulite facies metamorphism (1.8 GPa, 815°C) at circa 380–375 Ma. Cover rocks of the dome‐mantling The Straits Schist records peak conditions of ~1 GPa, 700°C at 386 ± 6 to 380 ± 4 Ma. Garnet breakdown to monazite and/or xenotime occurred in all units at circa 375–360 and 345–330 Ma. Peak Acadian metamorphic pressures increase systematically from the structurally lowest to highest units (from 1.0 to 1.8 GPa). This inverted metamorphic sequence is incompatible with the diapiric and fold interference models, which predict the highest pressures at the structurally lowest levels. Based upon P‐T‐t and structural data, we prefer a model involving, first, circa 380 Ma thrust stacking followed by syn‐collisional orogen parallel extension, ductile flow, and rise of the domes between 380 and 365 Ma. Garnet breakdown at circa 345–330 Ma is interpreted to reflect further exhumation during collapse of the Acadian orogenic plateau. These results highlight the power of integrating petrologic constraints with paired geochemical and geochronologic data from multiple chronometers to test structural and tectonic models and show that syn‐convergent orogen parallel ductile flow dramatically modified earlier accretion‐related structures in New England. Further, the Gneiss Dome belt documents gneiss dome development in a syn‐collisional, thick crust setting, providing an ancient example of middle to lower crustal processes that may be occurring today in the modern Himalaya and Pamir Range.

A COMBINED MODEL OF THE DIAPIRIC AND COLLISIONAL FORMATION MECHANISM OF THE PALEOPROTEROZOIC GRANITE-MIGMATITE-GNEISS DOMES OF THE SVECOFENNIAN BELT

Geological examples of the development of thermal dome structures in the Svecofennian belt demonstrate the relationship between plutonic and metamorphic events and metamorphic strengthening towards the core parts of the structures. The corresponding 2D geological model has been created to make a quantitative assessment of the effect of mantle magmas temperature at the base of the crust and the formation of the diapiric cores surrounded by high-temperature areas with a degree of granulite facies metamorphism. Modeling shows that there is a possibility of melting of the lower crust in the presence of a water fluid under the influence of mantle magmas. After melting, there occurs an ascent of partially molten material towards the upper crustal levels. An absorbed aqueous fluid changes to hydrous melt, thus lowering its viscosity and density. The ascending height of high-temperature cores is determined by the depth of viscous-to-elastoplastic transition in the crustal matter rheology. These materials ascend to upper levels in a partially molten state in the process of overthrusting due to "collisional" tectonics.

Xenocrystic zircon in Central New York State Kimberlites record Neoproterozoic disturbance of Grenville Basement rocks

Zircon grains from two central New York State (USA) Mesozoic kimberlite dikes intruding Middle and Upper Devonian sedimentary rocks were analyzed for UPb and LuHf isotopes, and trace elements, and were derived from different parts of the continental crust and/or upper mantle. Zircon from the Six Mile Creek (SMC) kimberlite has variable physical properties and “felsic” chondrite normalized REE patterns, and yields dates (321–1732 Ma) compatible with entrainment of sedimentary xenoliths during accent through the upper crust. In contrast, zircon from the Taughannock Creek (TAG) kimberlite have near concordant UPb dates that are distributed between intercept ages of 572 ± 25 and 960 ± 20 (2σ) Ma (MSWD = 0.61) with homogeneous LuHf systematics and 176Lu/177Hf ratios <0.0001. Examination in CL reveal TAG zircon grains vary in internal zoning and structures, but they are geochemically and isotopically similar. Their evolution includes early Neoproterozoic (ca. 950 Ma) granulite-facies metamorphism in equilibrium with garnet growth (flat to negative sloping HREE patterns and Ti-in-zircon temperatures >900 °C) during the final assemblage of the supercontinent Rodinia, followed by subsequent Pb-loss, resulting in a discordant array and decoupling of the UPb and LuHf isotopic systems prior to entrainment. Concordant zircon UPb dates, anchoring the lower intercept, yield an age of 596 ± 7 (2σ) Ma for the timing of Pb-loss. This most likely occurred in a zone of rifting and mafic magmatism beneath Central New York and Pennsylvania during the opening of the Iapetian Ocean.

P–T–t conditions of Early Palaeozoic low‐P high‐T granulite facies metamorphism in the southern Truong Son Belt, Central Vietnam

AbstractHigh‐grade metamorphic rocks are widely exposed along the SE–NW‐ to E–W‐trending shear zones in the Truong Son Belt, Central Vietnam, but few petrological studies have been conducted in this area. Herein, we report the occurrence of mylonitized granulites that crop out along the Dai Loc shear zone in the southernmost Truong Son Belt. Detailed petrographic analysis, geochemistry and P–T–t estimates of the evolution of two granulite samples are presented to elucidate the formation processes of these high‐grade metamorphic rocks. The results indicate that the rocks underwent two distinct metamorphic cycles. The first cycle (M1) is characterized by coarse‐grained granulite mineral assemblages, defining a tight clockwise P–T path with near‐isobaric heating to a near ultrahigh‐temperature peak at low pressure, followed by cooling. The prograde mineral assemblage (M1a) is indicated by inclusions of cordierite + sillimanite + biotite + quartz + spinel ± plagioclase in coarse‐grained garnet, orthopyroxene and cordierite. The mineral assemblage of garnet + orthopyroxene + cordierite + plagioclase + K‐feldspar + ilmenite + melt ± biotite (M1b) defines the peak P–T conditions of 5.3–6.3 kbar and 850–920°C. Post‐peak cooling (M1c) is marked by the formation of quartz + biotite symplectites around garnet and orthopyroxene. The second cycle involved medium‐pressure amphibolite facies metamorphism (M2), characterized by domainal development of fine‐grained kyanite‐bearing mineral associations. Petrographic observations indicate that these fine‐grained associations were formed during mylonitization. Zircon U–Pb dating reveals that the timing of granulite facies metamorphism appears to be coeval with the intrusion of a post‐collisional granitoid at 430–410 Ma. Granulite facies metamorphism and crustal melting were probably driven by asthenospheric mantle upwelling triggered by slab breakoff during the Early Palaeozoic. Considering previous structural and geochronological studies, the second metamorphic event likely occurred during the Triassic Indosinian orogeny.

Timescales and mechanisms of felsic lower continental crust formation: Insights from U-Pb geochronology of detrital zircon (Malenco Unit, eastern Central Alps)

Burial of supracrustal sedimentary material is one of the processes that contribute to the formation of the lower continental crust. We investigate the rate of this process in Permian lower continental crust from the Malenco Unit (eastern Central Alps) by constraining the age of protolith deposition and subsequent high-grade metamorphism of felsic, garnet-rich granulites. Detrital zircon cores, many of which are small (<50 μm), were dated by SIMS and yield concordant dates ranging from 387 ± 21 Ma (2σ) to 2380 ± 16 Ma, with the vast majority of the dates falling in the range 450–1000 Ma. For the seven cores in each sample that gave the youngest concordant dates, replicate analyses were carried out. None of these cores had all measured U-Pb dates agree within error, indicating significant disturbance by the Permian granulite facies metamorphism. A 206Pb/238U age of 424 ± 6 Ma for a core with two overlapping replicate analyses is the best available constraint on the maximum depositional age of the sedimentary protolith. This maximum depositional age also coincides with the first significant population in the probability density plot of detrital core dates, which is a double peak at ∼425 Ma and ∼450 Ma.Zircon detrital cores are separated from metamorphic overgrowths by a small seam of zircon that is riddled with tiny inclusions of quartz, biotite and muscovite. The first generation of metamorphic zircon rims has a 206Pb/238U age of 272.9 ± 2.8 Ma, displays steep heavy rare earth element (HREE) patterns and a moderate negative Eu-anomaly, and returns Ti-in-zircon temperatures of 740–780 °C. The second generation of metamorphic rims has a 206Pb/238U age of 263.8 ± 2.6 Ma, a flat HREE pattern and a pronounced negative Eu-anomaly, and gave Ti-in-zircon temperatures of 780–810 °C. Collectively these observations indicate that the first zircon rim formed on the prograde path at the onset of partial melting where muscovite was present but the rocks contained little garnet and no K-feldspar. Therefore, the metapelites resided at lower amphibolite facies, subsolidus conditions up until the intrusion of a Permian gabbro, which was emplaced shortly after the formation of the first generation of metamorphic zircon and caused heating to granulite facies conditions and widespread partial melting. There is no record of high-pressure metamorphism preceding granulite facies conditions, indicating that felsic lower crust in the Malenco unit was not formed by relamination.The Malenco unit was situated at the north-eastern active margin of Gondwana 420 Ma ago. Subduction-related accretion of sediments at lower crustal levels shortly after their deposition could explain the formation of this felsic lower crust that resided at subsolidus conditions for up to 100 My prior to Permian extension, gabbro intrusion and granulite facies metamorphism.

Granulite facies metasedimentary rocks: Insight into the formation of the East Kunlun continental arc

The metamorphism and dynamic processes of the Early Paleozoic continental margin arc in the East Kunlun orogen are still unclear. This study investigated the metasedimentary rocks with granulite facies in the Qingshuiquan area in the eastern section of the East Kunlun orogen. The kyanite is not found in the pelitic granulite. The phase equilibrium modeling indicates a middle-pressure type with metamorphic conditions of 0.7 to 0.8 GPa and 800 to 840°Cfor pelitic granulite. UPb dating of zircons from metasedimentary rocks yielded metamorphic ages of 490-480 Ma and detrital zircons ages of 1700-1500 Ma. REE patterns of the metasedimentary rocks are enriched LREEs and distinct negative Eu anomalies, the εNd values vary from −1 to −16. These results imply that the Precambrian supracrustal rocks were buried into the middle-lower crust level and granulite facies metamorphism occurred in Late Cambrian-Early Ordovician.Synchronous (520-480 Ma) magmatic rocks in Qingshuiquan and adjacent areas indicate that subduction of the Proto-Tethyan oceanic crust led to the formation of arc magmatic rocks. Meanwhile, it induced some sedimentary rocks to underthrust or return flows into the middle-lower crust of the upper plate and underwent granulite facies metamorphism.

Tectonic Evolution of the JLJB, North China Craton, Revisited: Constraints from Metamorphism, Geochemistry and Geochronology of the Ji’an Group and Related Granites

The Jiao-Liao-Ji Belt (JLJB) is the most representative Paleoproterozoic orogenic belt in the North China Craton (NCC). The sedimentation, metamorphism and magmatism of the Ji’an Group and associated granites provide significant insights into the tectonic evolution of the JLJB. In this study, we have synthesized published geochemistry and geochronology data on metasedimentary, metavolcanic and igneous rocks. According to the available data, the protoliths of the metasedimentary rocks are sets of shale, wacke, arkose, quartz sandstone and carbonate, while the protoliths of the metavolcanic rocks are calc-alkaline basalt, basaltic andesite, andesite, dacite and rhyolite. The rock assemblages indicate a transformation of the tectonic environment from a passive margin to an active continental margin following the onset of plate convergence and subduction. The A2-type gneissic granite (Qianzhuogou pluton) is formed in a subsequent back-arc basin extension setting at 2.20–2.14 Ga. The Ji’an Group was finally deposited in an active continental margin during the closure of a back-arc basin at 2.14–2.0 Ga. Then, the sediments were involved in a continent–arc–continent collision between the Longgang and Nangrim blocks at ~1.95 Ga. This process was accompanied by HP granulite-facies metamorphism at ~1.90 Ga. The subsequent exhumation and regional extension resulted in decompression melting during 1.90–1.86 Ga, producing metamorphism with an isothermal decompression clockwise P–T path. The resulting metapelites are characterized by perthite + sillimanite, and mafic granulites are characterized by orthopyroxene + clinopyroxene. The S-type porphyritic granite (Shuangcha pluton) is formed during the crustal anatexis. Meanwhile, extensive anatexis produced significant heating and triggered prograde to peak metamorphism with an anticlockwise P–T path. Cordierite-bearing symplectites around the garnet in the metapelites indicate a superposed isobaric cooling metamorphism. The ages of monazites and anatectic zircons suggest that the post-exhumation cooling occurred at 1.86–1.80 Ga. The Paleoproterozoic magmatism, sedimentation and metamorphism suggest a process of subduction back-arc basin extension and closure, collision and exhumation for the tectonic evolution of the JLJB.

An extended Neoarchaean to Neoproterozoic history of the Sandmata Complex (Aravalli Craton, northwestern India): Insights from metamorphic evolution and zircon-monazite geochronology of high-grade quartzofeldspathic gneisses

The Sandmata Complex in the Aravalli Craton of northwestern India experienced syn- to post-collisional high-grade metamorphism and migmatization related to the 1.9–1.7 Ga Aravalli orogeny. Garnet-bearing quartzofeldspathic gneisses are an important rock type in the Sandmata Complex. Their prograde assemblage is represented by garnet cores and inclusion minerals Bt + Pl + Qz, whereas the assemblage Pl + Kfs + Qz + Btmatrix + Sil + Ilm, along with chemically homogeneous garnet porphyroblasts, represents peak metamorphism. Post-peak retrograde metamorphism is reflected in the growth of muscovite, chlorite and biotite, and diffusion adjustments in garnet rims. Mineral paragenesis, mineral chemistry, conventional thermobarometry, and phase equilibrium modelling indicate P–T conditions of 6.7–10.2 kbar at 725–830 °C for peak metamorphism, and 4.8–6.9 kbar at 450–610 °C for post-peak retrogression. The P–T conditions determined from the quartzofeldspathic gneisses define a near-isobaric cooling P–T path. In contrast, previously studied high-grade pelitic gneisses and metagranitoids from the Sandmata Complex define an isothermal decompression P–T path. U–Pb dating of magmatic zircon cores in the quartzofeldspathic gneisses provides the crystallization age of their felsic protoliths as ∼ 2.52 Ga. Zircon and monazite geochronology show that peak metamorphism occurred at ∼ 1.90–1.78 Ga and thus shortly preceded the emplacement of the Gyangarh-Asind igneous suite and Anjana granite, previously dated at ∼ 1.72–1.64 Ga. A model of oceanic crust subduction followed by continent–continent collision during the Aravalli orogeny can explain granulite facies metamorphism in the Sandmata Complex. The combined results of this and previous studies support the operation of modern-style plate tectonic processes involving subduction and continental collision during Palaeoproterozoic time. We also identify a younger (∼0.90–0.78 Ga) metamorphic episode, preserved mostly in recrystallized monazite grains elongated parallel to the final deformation fabric, likely related to the South Delhi orogeny.

Archean Phlogopite Peridotite from Gneissic Enderbites of the Bug Granulite–Gneiss Terrane (Ukrainian Shield): Compositional Features and Possible Mechanisms of its Formationn

The paper reports mineral chemistry, whole-rock major and trace element composition, structure, and geological setting of the phlogopite peridotite lens within the Bug granulite–gneiss terrane of the Dniester–Bug Province, Ukrainian Shield. Geochemical features of the studied peridotites and minerals indicate a complex evolution of the lens. The early stage is marked by the crystallization of olivine + spinel (Al-chromite) cumulus from melt of supposably picritic composition, and further crystallization of clinopyroxene. The melt shows signs of hybridization by host gneissic enderbite. Orthopyroxene crystallized later and frequently replaces clinopyroxene. The phlogopite cystallization is likely related to the fluid activity and the growth of potassium potential in the fluid. Granulite-facies metamorphism accompanied by ductile deformations affected the mineral asseblage and chemical compositions of peridotites and orthopyroxenites. At the final stage, the rocks and minerals experienced retrograde metamorphism and local foliation of the rocks.

Geochemistry of granulite facies BIFs from the Luis Alves Craton (Southern Brazil): Implications for Archean-Paleoproterozoic depositional and tectonic settings

Banded Iron Formations (BIF) developed strongly during the Neoarchean – Paleoproterozoic Eras linked with global environmental processes such as the Great Oxidation Event (GOE) and changes in geodynamics, sea level, geobiology, hydrothermal systems, and sediment deposition conditions. BIF development and distribution worldwide are also related to supercontinent cycles, being a powerful tool to understand the evolution of plate tectonics. In this paper, we used field-based geological-structural data, petrography, X-ray diffraction, and lithogeochemical analyses to perform the first characterization of Banded Iron Formations at the Luis Alves Craton (LAC), Southern Brazil. The LAC BIF are classified as oxide and silicate facies, both with granoblastic texture. Oxide facies consists of quartz and magnetite. Besides these minerals, silicate facies contains olivine and ferrosilite, evidencing granulite metamorphism. Based on major elements oxide facies has a hydrothermal signature, with low content of Al2O3 and TiO2, whereas silicate facies register a continental sediment contribution been more enriched in these elements, MgO, MnO, and CaO. Trace-elements also indicate the main source for both silicate and oxide facies, which is seawater with a major influence of low-T hydrothermal sources, besides continental sedimentation sources. The Ce/Ce × anomaly is true negative, representing oxidizing waters, which also may represent a shallow water column depth. The depositional environment is inferred as a shallow, restricted basin. Oxide facies represents sedimentation closer to hydrothermal vents and with weak influence from continental clastic sedimentation, while silicate facies represent sedimentation closer to an adjoining continental landmass. Oxide facies has more Algoma-like characteristics, such as higher Fe and Eu contents, Sm/Yb and Eu/Sm typical of higher hydrothermal fluid inputs, and low siliciclastic contribution. On the other hand, silicate facies resembles Superior-type BIF, with higher siliciclastic contribution, negative Ce/Ce × anomalies, and Eu/Eu* <1.8. In summary, we propose the LAC BIF is representative of a transitional environment from Algoma-to Superior-type. The first depositions of the LAC BIFs may have developed within an Algoma-like environment during the Neoarchean. The waning tectonic and volcanic activity during the Siderian shifted to a platformal environment, with shallower waters that may induce the deposition of carbonates typical of Superior-type deposits. The depositional basin may have been closed by the Rhyacian and those sediments were then pushed down to the lower crust and underwent granulite facies metamorphism.

Exhumation of an Archean Granulite Terrane by Paleoproterozoic Orogenesis: Evidence from the North China Craton

Abstract Granulites represent high-grade metamorphic rocks of the deep continental crust. The metamorphism and exhumation of granulites from Archean terranes provide insights into the crustal evolution of Archean cratons and shed light on the formation and reactivation of cratons. We present petrology, U–Pb geochronology (zircon, rutile, and titanite), and pressure–temperature (P–T) paths for metadiabase dikes in an Archean granulite terrane of the North China Craton. Garnet (Grt) coronae in the metadiabase dikes are developed between plagioclase (Pl) and clinopyroxene (Cpx) via the reaction Pligneous + Cpx → Pl1 + Grt ± quartz. The reaction proceeds inward within the plagioclase, progressively consuming Pligneous (XAn = 0.65–0.69) and leaving Ca-poor Pl1 (XAn = 0.46–0.53) as the residue. Geothermobarometry and P–T pseudosections suggest peak conditions for garnet formation at ~800°C and 10–13 kbar. During retrograde metamorphism, Grt broke down to Ca-rich Pl2 (XAn = 0.73–0.74), and ilmenite replaced rutile. Geothermobarometry and Zr-in-titanite temperatures constrain the P–T conditions of retrograde metamorphism at 700–750°C and 4.5–7.5 kbar. Zircon and titanite U–Pb geochronology shows that the protolith of the metadiabase dike was formed at 2.4 Ga and underwent granulite-facies metamorphism at 1.86 Ga. The intrusion of mafic dikes into the Archean granulite terrane indicates that the Archean basement was also heated and buried in the Paleoproterozoic. The metadiabase dikes and the hosting Archean basement underwent Paleoproterozoic granulite-facies metamorphism at a depth of ~40 km, followed by near-isothermal decompression and subsequent near-isobaric cooling (cooling rate of 1–3°C Myr−1) at depths of 15–25 km. Crustal shortening and thickening may have been caused by the underplating of the Khondalite series beneath the Archean basement during the amalgamation of supercontinent Columbia. The Paleoproterozoic orogeny induced a second generation of metamorphism of the Archean basement along the margin of the craton and drove the exhumation of the Archean granulite terranes to the middle crust (~15 km).

Mechanism and age of gold remobilization in orogenic gold deposit from primary laminated ore to the high-grade ore: A case study from Cenozoic Bangbu gold deposit, Tethyan Himalaya

The formation of high-grade gold ore has always been attractive, mostly located in Precambrian gold deposits with various mechanisms. Yet, in the orogenic Bangbu gold deposit, Tibet, we discovered that the coupled dissolution-reprecipitation (CDR) process and low melting point chalcophile elements (LMCE) -related melting are responsible for the formation of high-grade ore at ca. 38 Ma constrained by in-situ U-Th-Pb dating of hydrothermal monazite. Three generations of pyrite have been recognized, metamorphic barren pyrite (Py I) in the host schist, hydrothermal pyrite (Py II) in the preceding primary laminated ore with an Au grade of ∼8 g/t, and the hydrothermal coarse pyrite (Py III) in later massive ore with a grade of up to 29 g/t. In the primary laminated ore, the Py II in the sulfide ribbons is intergrown with native gold and colloform quartz and carries abundant trace element contents, such as Au, As, Cu, Pb, Zn, Te, Se, and Bi. These suggest that the rapid precipitation and local supersaturation occurred during the episodic crack-seal process. In the later high-grade massive ore, Py III exhibits distinct zonation, ranging from Py III-1 in the core to Py III-3 in the rim. The Py III-1, porous and rich in inclusions, has sharp boundaries and successive lattice orientation with the mantle Py III-2 suggesting a CDR process. Higher Au and As in Py III-2 than the Py III-1, and the close affinity between visible gold and LMCE melt inclusions at the boundary suggest that gold was remobilized and facilitated by the LMCE during the CDR process. The electron backscatter diffraction (EBSD) mapping result shows little crystallographic misorientations of Py III, excluding the deformation process that has significantly contributed to the remobilization of gold. The decrease of the δ34S ratio of pyrite between Py II (δ34S = 2.0‰ to 4.3‰, mean 3.4‰) and Py III (δ34S = −2.5‰ to 3.8‰, mean −0.2‰) most likely reveals a more oxidized condition in the open space which may trigger the CDR process and gold remobilization of the pyrite in the primary ore. Within the zonation of Py III, an increasing trend of the δ34S ratio represent the evolution of the redox state of fluids towards relatively reducing conditions during the healing of quartz veins. The newly obtained ca. 38 Ma age is comparable to regional amphibolite- to granulite-facies metamorphism during the continental subduction, which was considered to trigger the modification of primary gold orebody to form high-grade ore.